We report the effect of germanium as n-type dopant on the electrical and optical properties of AlxGa1-xN layers grown by plasma-assisted molecular-beam epitaxy. The Al content has been varied from x = 0 to 0.66, confirmed by Rutherford backscattering spectrometry, and the Ge concentration was increased up to [Ge] = 1×10 21 cm −3 . Even at these high doping levels (> 1% atomic fraction) Ge does not induce any structural degradation in AlxGa1-xN layers with x < 0.15. However, for higher Al compositions, clustering of Ge forming crystallites were observed. Hall effect measurements show a gradual decrease of the carrier concentration when increasing the Al mole fraction, which is already noticeable in samples with x = 0.24. Samples with x = 0.64-0.66 remain conductive (σ = 0.8-0.3 Ω −1 cm −1 ), but the donor activation rate drops to around 0.1% (carrier concentration around 1×10 18 cm −3 for [Ge] ≈ 1×10 21 cm −3 ). From the optical point of view, the low temperature photoluminescence is dominated by the band-to-band

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Radio‐frequency (RF) sputtering is a low‐cost technique for the deposition of large‐area single‐phase AlInN on silicon layers with application in photovoltaic devices. Here, the effect of the Al mole fraction x from 0 to 0.56 on the structural, morphological, electrical, and optical properties of n‐AlxIn1−xN layers deposited at 550 ºC on p‐Si(100) by RF sputtering is studied. X‐ray diffraction data show a wurtzite structure oriented along the c‐axis in all samples, where the full width at half maximum of the rocking curve around the InN (0002) diffraction peak decreases from ≈9° to ≈3° while incorporating Al to the AlInN layer. The root‐mean‐square surface roughness, estimated from atomic force microscopy, evolves from 20 nm for InN to 1.5 nm for Al0.56In0.44N. Low‐temperature photoluminescence spectra show a blueshift of the emission energy from 1.59 eV (779 nm) for InN to 1.82 eV (681 nm) for Al0.35In0.65N according to the Al content rise. Hall effect measurements of AlxIn1−xN (0 < x < 0.35) on sapphire samples grown simultaneously point to a residual n‐type carrier concentration in the 1021 cm−3 range. The developed n‐AlInN/p‐Si junctions present promising material properties to explore their performance operating as solar cell devices.

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In this paper, we study intersubband characteristics of GaN/AlN and GaN/AlGaN heterostructures in GaN nanowires structurally designed to absorb in the mid-infrared wavelength region. Increasing the GaN well width from 1.5 to 5.7 nm leads to a red shift of the intersubband absorption from 1.4 to 3.4 μm. The red shift in larger quantum wells is amplified by the fact that one of the GaN/AlN heterointerfaces (corresponding to the growth of GaN on AlN) is not sharp but rather a graded alloy extending around 1.5-2 nm. Using AlGaN instead of AlN for the same barrier dimensions, we observe the effects of reduced polarization, which blue shifts the band-to-band transitions and red shifts the intersubband transitions. In heavily doped GaN/AlGaN nanowires, a broad absorption band is observed in the 4.5-6.4 μm spectral region.